The present invention in general relates to improvements made to solenoid valves. It is more particularly applicable to solenoid valves comprising an electromagnet subassembly associated with a pneumatic switching subassembly with a valve plug, the pneumatic switching subassembly being composed of a body enclosing the valve plug trapped between two sealing seatings facing each other. The valve plug movement is provoked firstly by a spring located in the mobile core of the electromagnet subassembly by means of a pusher, and secondly by means of a spring located under the valve plug (usually called valve plug underspring) and bearing around the lower seating of the pneumatic switching subassembly, this valve plug underspring forcing the said valve plug into contact with the upper seating of the pneumatic switching subassembly when the solenoid valve is energized. In the rest position, in other words when the solenoid valve is not energised, the spring of the mobile core of the electromagnet subassembly pushes the core into contact on the pusher and forces the valve plug onto the seating of the lower channel of the solenoid valve.
The improvements according to the invention will be better understood with reference to FIGS. 1 and 2, which show axial and vertical sections through an electromagnet according to prior art.
These FIGS. 1 and 2 show that this invention relates to a solenoid valve comprising firstly an electromagnet subassembly denoted as a whole as reference 10, and secondly a pneumatic switching subassembly denoted as a whole as reference 12. Mechanical means are used to assemble these two subassemblies 10 and 12.
The pneumatic switching subassembly 12 is composed of a body enclosing a valve plug 14, preferably made from an elastomer, that is trapped between two sealing seatings 15 and 16 facing each other. The movement of the valve plug 14 in the body of the pneumatic switching subassembly 12 is caused firstly by a spring 17 positioned in the mobile core 18 of the electromagnet subassembly 10 by means of a pusher 19, and secondly by a spring 20 (valve plug underspring) to return the valve plug.
In the rest position, in other words when the electromagnet is not energised, the spring 17 pushes the mobile core 18 such that it forces the valve plug 14 through its pusher 19 onto the seating 16 of the lower channel of the solenoid valve.
The force applied by the spring 17 of the mobile core 18 must be sufficient to guarantee leak tightness of the lower seating 16, and to achieve this result, this force must be equal to the following sum:
(pressurexc3x97cross section of seating 16)+force of spring 20+additional sealing force. 
In this respect, refer to FIG. 3 which shows the force of the spring 17 as a function of the air gap of the lower seating 16 of the valve plug.
When the solenoid valve is ni the open position, in other words when the electromagnet is energised, the magnetic force that it generates in the mobile core 18 is greater than the force of the spring 17 under these conditions, the mobile core 18 forces it into contact with the fixed core 21a (or the yoke) of the electromagnet subassembly 10 and it no longer applies any force on the valve plug 34 through the pusher 19. The return spring 20 of the valve plug 14 then forces the valve plug into contact with the upper seating 15 of the subassembly 12.
Considering its manufacturing mode (particularly the steel quality, wire diameter, number and pitch of the turns, free length), the spring 17 of the mobile core can only be made with a certain tolerance on the free length, and therefore on the force at a certain predetermined height called the xe2x80x9cplaced heightxe2x80x9d. In solutions used at the moment, the force can vary from 20 to 40% for a load at a placed height, which is becoming less acceptable in the current attempt to improve the performances and miniaturization of this type of solenoid valve.
The most frequently used solution to solve this problem at the moment is to sort the springs in order to reduce the manufacturing tolerance. This technique is a simple means of reducing manufacturing dispersions, but it still leaves relatively large variations and it increases the cost of the spring production due to manufacturing scrap.
Another solution used at the moment is to adjust the mobile core spring after the solenoid valve has been assembled. Consequently, the yoke, in other words the fixed core of the electromagnet subassembly, is perforated and drilled in the axial direction so that a threaded pusher can be fitted into it on which an O-ring is fitted to make it leak tight. This threaded pusher is in contact at one of its ends with the spring of the mobile core of the electromagnet subassembly. In order to adjust the solenoid valve, in other words the spring force on the mobile core, all that is necessary is to apply the maximum usage pressure in the lower channel of the solenoid valve and then screw the pusher in until the spring of the mobile core seals the lower seating under the pressure. This known solution is precise because the mobile core spring only applies the force necessary to create a seal on the lower seating of the valve plug of the solenoid valve. The disadvantage of this solution is its manufacturing and installation costs: it requires tapping to be formed in the fixed yoke of the solenoid valve, machining of an additional part (threaded pusher) and the installation of an additional O-ring to seal the assembly.
Starting from this state of the art, the present invention has the objective of solving the technical problem mentioned above, in other words:
to reduce dispersions in the force in the spring in the mobile core of the electromagnet assembly, and
also to compensate for dispersions in the valve plug return spring (valve plug underspring) of the pneumatic switching subassembly, in a simple manner without it being necessary to use additional sealing means.
Consequently, this invention relates to a process for calibration of the mobile core spring of a solenoid valve comprising an electromagnet subassembly associated with a pneumatic switching subassembly with a valve plug, the valve plug being moved in the body of the pneumatic switching subassembly firstly by a spring located in the mobile core of the electromagnet subassembly through a pusher, and secondly by a spring located under the valve plug and bearing on the lower seating of the pneumatic switching subassembly, this process being characterised in that:
a force sensor is put into position, either at a predetermined distance from the upper face of the core, this distance being equivalent to the actual travel distance of the mobile core, or directly in contact with the face of the core when it is required to adjust the spring based on its force after the travel distance (core in contact on the yoke);
the spring is installed in a reaming provided for this purpose in the axis of the mobile core, and then,
an elastic pin is put into position behind the said spring, that is forced more or less into the reaming of the core while measuring the force generated by the spring on the sensor, and
the pin is no longer forced into the core as soon as the required force is measured by the sensor.
According to one embodiment of the process according to the invention, the parameters used for adjustment of the spring are firstly:
the placed spring height, in other words the height at which the spring must supply the force necessary to seal the lower seating, and
secondly, the force necessary to seal the lower seating, these two parameters being either calculated or measured.
According to another embodiment of the invention, the said pin is perforated on each side. It may preferably be made by spiral winding of a metallic wire, or it may be in the form of a cylindrical part or a ball.
The process according to the invention may be applied to a normally open type solenoid valve. According to this particular application, the spring of the mobile core of the electromagnet subassembly is adjusted by using the sensor to measure the force generated by the valve plug underspring when it is assembled in the pneumatic switching subassembly and this force is reproduced in the mobile core spring so that the tolerances of the two springs are compensated by the setting of the mobile core spring.